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Outline of landforms and geology of Japan
Geology
Fig.
22
Simplified geological map [
]
Photo 1:
Complex
structure of strata []
Photo 2:
Crystalline schist (Nagatoro) []
Fig. 23
Basement geologic map of Japan []
Fig. 24 Cross section of southwest Japan []
Rocks in the Japanese Islands are roughly classified into pre-Neogene rocks, plutonic rocks including granite, and Neogene and Quaternary rocks. Pre-Neogene rocks constitute the Japanese Islands as basement rocks, covered with Neogene and Quaternary deposits and volcanic products. Plutonic rocks intruded these rocks. Strata subjected to intense force have complex structures such as folds and faults. Horizontal strata are only seen in younger deposits in Japan.
Basement -- accretionary complexes and metamorphic rocks
Basement of the Japanese island arcs mainly comprises accretionary complexes produced from the Cambrian to the Paleogene and regional metamorphic rocks.
Accretionary complexes generally include basalt, pelagic chert consisting mainly of radiolarian shells, siliceous mudstone, and thick sandstone and mudstone. Chert of various ages and basalt are contained in muddy matrices as fragments of all sizes (mélange) (see “Accretionary prisms”). In addition, accretionary complexes may have blocks of basalt and reef limestone derived from seamounts, and ophiolite. The seamounts were basaltic volcanoes with reef limestone on top, formed on the seafloor far from a subduction zone and moved to a trench. Ophiolite is regarded as fragments of an oceanic plate and the upper mantle, consisting of ultramafic rocks (serpentinite), mafic rocks (gabbro, dolerite, and basalt), and pelagic sediments such as chert.
Metamorphic rocks regionally distributed are metamorphosed accretionary deposits, mainly crystalline schist characterized by the parallel arrangement of the constituent minerals (schistosity). Gneiss also occurs in metamorphic rock zones. Metamorphic rock in island arcs is briefly described in the section, “Metamorphic rock”.
These basement rocks are divided into some units based on their formation ages and geological characteristics. These units are zonally distributed in order of which their ages become younger oceanward, indicating that the Japan Islands have been developed oceanward in the subduction zone. Jurassic accretionary complexes most widely occupy the Japanese Islands. Most of the units are bordered by thrust faults; an older unit overlies the adjacent younger unit. (See also “Japan in a subduction zone” and “Formation history of the Japanese Islands”).
The characteristics of basement units are different in southwest Japan, northeast Japan, and Hokkaido.
[Southwest Japan (from Kanto to Kyushu)]
In southwest Japan, the geologic structure formed in the subduction
zone are well preserved. Basement rock units are distributed clearly
parallel to the Nankai Trough in order of age. However, the Chichibu
Belt, a Jurassic accretionary complex zone is situated on the Pacific
side of the Median Tectonic Line, to the south of the Mino-Tamba Belt
which is also the Jurassic accretionary complex zone. The Chichibu Belt
is considered a nappe which is allochthon that migrated from the
birthplace by thrusting. See also “200 to 140
million years ago (Jurassic)" (Formation history of the Japanese
Islands).
Figure 24 shows a cross section of southwest Japan. This cross section
was made on the basis of seismic profiling data as well as geological
surveys of exposures. The profile is markedly different between the
inner and the outer zones bordered by the MTL which cuts the crust northward
at an angle of 40 degrees. The inner zone (the Sea of Japan side) is
characterized by almost horizontal structure. The uppermost layer is a nappe group of acrretionary complexes such as the Mino-Tamba Belt and
the Maizuru Belt. Granitic batholiths underlie the uppermost layer and
granite intrudes into the layer, but the seismic profiling data suggests
geologic body of which the components are unknown under the nappe group
layer in the Chugoku region. The lower crust is underneath the granitic
batholiths (Ito et al., 2010).
In the outer zone, the upper crust is occupied with the Shimanto
accretionary complex gently dipping northward and the lower crust is
extremely thin. The Shimanto and younger accretionary complexes intrudes
into the inner zone. The Sambagawa metamorphic rocks are distributed
along the MTL, traced to a depth of about 20 kilometers.
The characteristics of each basement rock unit are briefly described as
follows.
